The separation of electrolytes (salts, alkalis, acids) and water from organic phases, for example water-immiscible polymers, aromatics, aliphatics, plays an important role in industrial processes. According to the prior art, such separation has been carried out by means of centrifuging, deposition, distillation, spray drying, evaporation-extrusion, precipitation and/or fiber coalescence. In general, the separation of the electrolytes (alkali and acid) is not carried out until after neutralization to form the corresponding salts, with acids and alkalis.
A disadvantage is the consumption of neutralizing agents and the concomitant loss of alkali and/or acid, as well as disposal/use of the salt. Other disadvantages of the methods described above for separating an electrolyte-free (alkali, acid, salt) organic phase are that they necessitate on the one hand extensive use of energy in order to separate the water and, on the other hand, elaborate separation of residual contents of electrolytes (salts) by washing processes. This entails high costs owing to the associated water treatment and the high energy demand. Tests to improve the phase separation, for example temperature variation, improved centrifuges or reducing the size of the separators by using coalescence aids, often lead to insufficient separation or a strong susceptibility to clogging of the coalescence aids by build-up of salt.
In the specific case of the polyether polyol production process, the salts and residual water contents are separated from the polyol phase according to the prior art by centrifuging, deposition or distillation and filtration (see, for example EP 0 038 986 A2).
On an industrial scale, polyether polyols are usually produced by addition of alkylene oxides, in particular polypropylene oxide and/or ethylene oxide, to starting compounds with acidic hydrogen atoms (for example water, polyalkylenes or polyamines) in the presence of basic substances, generally alkali metal hydroxides such as KOH or NaOH, as a catalyst. In one of the processing methods which is customary at present, the basic catalyst (for example KOH) is removed from the alkaline polymerizate in a plurality of method steps. First, the alkaline polymerizate is neutralized e.g. with dilute sulfuric acid, after which the majority of water is distilled off with simultaneous crystallization of the inorganic salt (here K2SO4). The precipitated salt is filtered off, whereupon the residual water is distilled off and the residual amount of salt is removed by filtration.
The disadvantages of these known neutralization methods are, on the one hand, the consumption of neutralization acid and very high energy consumption for distilling the water. On the other hand, it is difficult to filter off the usually very finely divided salt.
The polyether polyol mixture obtained after the polymerization consists of an organic phase, which contains polyether polyol and the by-products created during the reaction (inter alia 1,4-dioxane, 2,5-dimethyl-1,4-dioxane, 2,4-dimethyl-1,3-dioxalane, 2-ethyl-4-methyl-1,3-dioxalane, 2-methyl-2-pentanal, acetaldehyde, acetone, allyl alcohol, allyloxipropanol, DPG allyl ether, ethylbenzene, ethylene, ethylene oxide, methanol, propionaldehyde, propylene oxide and/or toluene). Water is dissolved in this phase, the extent of which depends on the type of polyether polyol, i.e. the molecular weight or C chain length and the proportion and distribution of ethylene oxide and propylene oxide. The aqueous phase contains salt, which may contain ions of the alkali and alkaline-earth metal group, for example Li, Na, K, Be, Mg, Ca, Sr, Ba and creates for example H2SO4, HCl, H3PO4, HNO3 or CO2 during the treatment or neutralization with acids. A water-immiscible aromatic or aliphatic solvent (for example toluene or hexane) may furthermore be used in order to improve the phase separation.
In the specific case of producing polycarbonate, copolycarbonate or polyester carbonate according to the so-called phase interface method (cf. phase interface method for polycarbonate production, see Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH Verlag), dihydroxyarylalkanes in the form of their alkali metal salts are reacted with phosgene in the heterogeneous presence of inorganic bases such as sodium hydroxide or an inorganic solvent (for example chlorobenzene and/or methylene chloride), in which the polycarbonate product is highly soluble. During the reaction, the aqueous phase is distributed in the organic phase and after the reaction, the organic phase containing polycarbonate is washed with an aqueous liquid and neutralized with acids (for example HCl), so that inter alia electrolytes (for example sodium chloride, sodium carbonate, optionally sodium sulphate) and traces of unreacted raw materials (for example phenol, isooctylphenol, ethyl piperidine and bisphenol) are removed, and the washing liquid is subsequently separated. The aqueous phase is conventionally separated by spray drying, evaporation-extrusion, precipitation of the polycarbonate, centrifuging (see EP 264 885 A2) or by fiber coalescence (see DE 19 510 061 A1).
In general, salts constitute contamination and must be separated from the organic product (for example polycarbonate or polyether polyol).
The object can be achieved by a novel filtration method in which both water and electrolytes (alkalis, acids, salts) are retained, and only the organic phase optionally with residues of physically dissolved water is let through organophilic filters.
Filtration methods are conventionally used for solid-liquid separation, for example to separate particles. They have the disadvantage that they are not capable of separating finely distributed dispersions or emulsions of water and salt in organic solvents (products) from one another. Conventional membrane filtration methods by means of polymer membranes (for example of polyether sulfone, polysulfone, polyamide, cellulose acetate) generally separate the aqueous phase from the organic-aqueous mixture, so that the filtered aqueous phase contains the electrolytes and is generally the desired product, and the organic phase often contains residual electrolytes and must therefore be treated separately or is disposed of. They furthermore have the disadvantage of low chemical stability with respect to organic compounds and a high temperature sensitivity.
Another way of separating the phases from one another is to use membranes which separate water selectively (see Verfahren zur Pervaporation oder Dampfpermeation; see Melin, Rautenbach—Membranverfahren—Springer Verlag 2004). In this case, the aqueous phase penetrates through the membrane and the organic phase is retained. In the case of salt-containing solutions, precipitation of the salts takes place in the region of the separating and support layers of these membranes, so that these membranes are susceptible to clogging. At the same time, these membranes exhibit a low permeation flux.
Feng et al. have furthermore described the use of superhydrophobic coated sieves for separating diesel oil and water. (Feng, L., Zhang, Z., Mai, Z., Ma, Y., Liu, B., Jiang, L. & Zhu, D., A Super-Hydrophobic and Super-Oleophilic Coating Mesh Film for the Separation of Oil and Water. Angewandte Chemie, 116 (2004) 2046). Besides production of the superhydrophobic coated sieves, Feng et al. describe the influence of the sieve mesh width on the hydrophobicity of the sieve. The superhydrophobicity is described with the aid of water drops and diesel oil drops. The separation of diesel oil-water is described as a possible field of use, the liquids being, however, explicitly described as non-emulsified. The work by Feng relates exclusively to the production of the superhydrophobic sieve for separating water and diesel oil. The separation of salt-containing or electrolyte-containing aqueous phases is not described and carried out.
It is therefore an object of the present invention to enable or improve the separation of an electrolyte-free organic phase from an electrolyte-containing (salt-containing) aqueous and organic phase.